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The present invention generally relates to the field of optical measurements. In particular, the present invention relates to a quantum optical system for the characterization of polarization mode dispersion in an optical system and the method of performing the characterization. The invention also relates to performing said characterization on an active (viz., in-use) optical communications channel.
As the demand for increased bandwidth continues, telecommunications providers are looking for new ways to provide the additional bandwidth. The ultimate bandwidth available in an optical communications channel (e.g., an optical fiber) is limited by its optical properties. In particular, if the transmission time of an optical pulse through an optical channel is dependent on its polarization, the channel is said to exhibit polarization mode dispersion (PMD). PMD results from the birefringence of optical materials in the optical path which leads to a differential propagation delay between the orthogonal polarization components of light transmitted through the optical channel. PMD limits the bandwidth of the optical channel because it broadens the optical pulses and increases the bit error rate (BER). As modulation speeds increase, pulse durations decrease, and accurate compensation of PMD are required to maintain a low BER. To control such compensators, precise characterization of the PMD of the optical channel is required. Additionally, the PMD of an optical channel depends on the wavelength of the propagating light. Presently, optical communications fiber are wavelength multiplexed. That is, one physical channel is used to carry many communications channels, where each communications channel is identified uniquely by the wavelength of the light it uses. Thus, in addition to accurate and precise PMD characterization, the co-temporal characterization of the PMD of each of the multiplexed wavelengths in the channel is required.
Prior art methods of characterizing PMD have depended on classical optical (as opposed to quantum optical) phenomena. For example, the NetTest NEXUS Polarization Mode Dispersion Measurement System employs a Michelson interferometric technique to analyze PMD. Essentially these prior art systems attempt to measure the amplitude and relative phase of the two vector components of the polarized light. Other prior art systems use an optical signal analyzer (OSA) to measure the effects of PMD (that is, the system measures power variations at a fixed set of output polarization states as function of wavelength). In the former case, the light that has passed through the device under test must be divided into two arms of an interferometer, potentially introducing non-common path errors, while with the second approach the dispersive phase delay is not measured directly, it being inferred from the measured intensity variations.
One object of the present invention is to provide an apparatus that uses quantum-optical phenomena to measure the effective time delay between polarization states of light the have propagated through an optical element. A second objective of the invention is to provide a method of performing PMD characterization on an optical element. A further objective of the invention is to provide a PMD characterization apparatus that may be used on an active communications channel, that is, in the presence of signal photons. Yet another objective of this invention is to provide a method of characterizing PMD in an active communications channel.
The present invention relates to an apparatus and method for determining the PMD of an optical element and specifically of an optical communications fiber. The method includes the generation of a beam of xe2x80x9ctwinonsxe2x80x9d. Twinons are a pair of quantum mechanically entangled photons, typically emitted from a parametric down conversion optical process. Each photon in a twinon has a corresponding twin photon that is correlated with it in frequency (or energy), direction (or momentum) and polarization. Each of these photons loses its individuality when it becomes one half of an entangled pair. In the invention, the apparatus is arranged such that each of the twin photons travels in substantially the same direction but differ in wavelength and polarization state. Specifically, the twin photons in this invention have orthogonal polarizations. Although every twinon in this invention has substantially the same total energy, each of the two twin photons generally has a different, random energy, within a range of energies determined by the configuration of the parametric down conversion.
The key to the invention is understanding that each twinon is a single entity that happens to be made up of two photons. The behavior of one photon is correlated with the behavior of the other, even when they appear to be in separate locations. When a twinon traverses an optical system in which there are multiple indistinguishable paths, quantum optical interference determines in which of the paths the photons will be detected. For example, destructive interference can prevent two different detectors from observing a photon simultaneously while quantum optical constructive interference can xe2x80x9cforcexe2x80x9d one photon to appear at each detector. Thus, in the absence of any differential delay (viz., PMD) between the two orthogonally polarized twin photons, quantum-interference effects can either eliminate or reinforce coincident detections (xe2x80x9cCD""sxe2x80x9d) on two separated detectors.
In this invention, the twinon beam propagates through the optical element or device under test (DUT) and then impinges on a beamsplitter at the input of a quantum-interferometric device (QID). Unlike classical interferometers, a QID does not bring two interfering photons together on a single detector. Instead, the two arms of the device each terminate at a separate detector.
In the presence of a polarization-specific delay (viz., PMD) the twinon acts like two un-entangled photons. In this case, as in classical optics, each photon may be reflected or transmitted at the beamsplitter. About half of the time one photon will propagate down one arm of the QID and one photon will propagate down the other arm of the QID. Thus, when the photons are acting independently (that is, when they are distinguishable) the CD rate is substantially one half the maximum observable rate.
In the invention, one arm of the QID includes a variable, polarization-specific delay element. When this inserted delay from this element exactly compensates for the PMD induced delay, the twin photons are within a coherence length of each other and quantum interference takes hold. Depending on the phase of the photons, the CD count rate either dips to near zero or rises significantly. The inserted delay for which rate of coincident detections exhibits its maximum change is a measure of the PMD.
One arm of the QID used in this invention also includes a wavelength demultiplexer and an array of detectors. The demultiplexer directs photons in different wavelength bands into individual detectors in the array. Comparing the output of each detector in the array with the single detector in the other arm of the QID generates a wavelength histogram of detection coincidences as a function of polarization-specific delay. As in the single wavelength case, the variable delay at which each wavelength channel sees the CD rate dip or peak is the PMD for that wavelength.
In one embodiment the system includes an entangled photon source which projects a beam into the optical element to be measured, a beam dividing element to divide the light exiting the optical element to be measured into two beams, a polarization-specific fixed delay element and a polarization-specific, variable delay element in one of the two beams, an optical demultiplexer in one of the two beams, a plurality of first detectors to detect the light emerging from the optical demultiplexer, and a second detector. The entangled photon source generates photon pairs, each of said pairs includes a first twin photon and a second twin photon that are correlated in time, wavelength and polarization. The beamsplitter defines a first optical path and a second optical path, the two optical paths being indistinguishable in the quantum-optical sense. The polarization-specific, variable delay element introduces a variable, differential time delay between the two orthogonal polarization states of the photons in the system. The fixed polarization-specific delay element provides a time delay bias between the two polarization states, thereby allowing the variable delay element to provide relatively negative and relatively positive time delays. The optical demultiplexer is designed such that photons with wavelengths in specific predetermined wavelength bands are directed into a plurality of spectral beams. Each of the plurality of the first detectors is positioned to receive one of the plurality of spectral beams and each is sensitive to the arrival of individual photons. The second detector, also sensitive to individual photons, is positioned to receive light from the beamsplitter along the second optical path. In one embodiment the system also includes a processor in communication with the plurality of first detectors and the second detector. The processor determines if a coincident detection of photons has occurred.
In a second embodiment the apparatus includes the polarization-specific fixed delay element in the second optical path. In another embodiment the polarization-specific, fixed delay element and the polarization-specific, variable delay element are both located in the twinon beam before the beam dividing element.
Yet another embodiment of the apparatus includes an optical signal injector component and an optical signal extractor component at the source end and QID end of the DUT, respectively. Said injector and said extractor combine or separate said twin photon beam from an optical communication signal using wavelength, temporal, or spatial multiplexing. This embodiment may also have a fast shuttering device to block the entrance of the QID.
In one embodiment the method includes the steps of forming a first twin photon and a second twin photon, and transmitting the first twin photon and second twin photon through an optical element. The method includes the additional steps of identifying coincidences in the detection of the twin photons at a first detector and at a second detector after transmission through the optical element, determining a wavelength of said one of said twin photons, adjusting the relative delay in the paths taken by said twin photons, and determining said polarization mode dispersion of said optical element in response to said steps of detecting.
A second embodiment of the method includes the steps of forming a first twin photon and a second twin photon, and transmitting the first twin photon and second twin photon through an optical element. The method further includes the steps of combining said twin photons with an optical communications signal prior to transmission through said optical element and separating said twin photons from said communications signal after transmission through said optical element. The method includes the additional steps of identifying coincidences in the detection of the twin photons at a first detector and at a second detector after transmission through the optical element, determining a wavelength of said one of said twin photons, adjusting the relative delay in the paths taken by said twin photons, and determining said polarization mode dispersion of said optical element in response to said steps of detecting.
A third embodiment of the method includes all the steps of the second embodiment and, in addition, the steps of time multiplexing said twin photon beam and said optical communications signal.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.